2013-04-26  Geoff Pike  <gpike@chromium.org>

gold/
	* gold.cc (queue_final_tasks): invoke layout->queue_build_id_tasks().
	* layout.cc (Hash_task): New class.
	(Layout::queue_build_id_tasks): New function.
	(Layout::write_build_id): Handle single-thread portion of build ID
	computation.  (In some cases, all of it is single-threaded.)  Replace
	{sha1,md5}_process_bytes with {sha1,md5}_buffer to get the same
	functionality in fewer lines of code.
	* layout.h (Layout::queue_build_id_tasks): New function declaration.
	* options.h (General_options): make "--build-id" default to tree
	rather than sha1.  Add two new options related to --build-id=tree:
	--build-id-chunk-size-for-treehash and
	--build-id-min-file-size-for-treehash.
	* Makefile.am: add testing of --build-id=tree and related new options
	(these tests will be invoked by "make check").
	* Makefile.in: Regenerate.